According to the incomplete statistics from United Nations Educational, Scientific, and Cultural Organization (UNESCO) and Food and Agricultural Organization (FAO), the total area of saline-alkaline land in the world is 954.38 million hectares, of which 99.13 million hectares are located in the People's Republic of China (PRC). The formation of alkaline soil and alkalized soil in China is mostly associated with the accumulation of carbonate in the soil, thus the degree of alkalization is generally high, and the plants can hardly survive in the area with severely salinized and alkalized soil.
A variety of saline-alkaline soils are formed under certain natural conditions, and the essence of the soil formation is mainly the redistribution of various soluble salts on the ground in the horizontal and the vertical direction, so that the salt content is gradually accumulated in the soil surface of the salt-collecting area. The root cause of the formation of saline-alkali soil is poor water condition, thus in the early stage of soil improvement, the efforts should be focused on improving the moisture condition of soil. At present, the treatment of saline-alkali soil is generally implemented in several steps, firstly draining and washing salt in order to reduce salt content in the soil; then growing plants resistant to salt and alkali so as to fertilize the soil; and finally planting crops. The specific improvement measures comprise: drainage, irrigation for washing salt, colmation and modification, planting paddy, improving fertility and modifying the soil, leveling land, and improving soil with chemicals. Such a method has a desirable effect on the treatment of land with low to moderate salinization and alkalization, but the treatment needs a long period and high cost, and there is a rebound phenomenon of salinization and alkalization. In addition, the method is not applicable to the secondary saline-alkali land caused by improper farming management, the coastal saline-alkali land formed by seawater impregnation, and the treatment of grassland saline land, which are also the bottleneck and difficulty in the treatment of salinized and alkalized soil treatment.
Humic acid is a kind of macromolecular and organic mixture with weak acidity formed with remains of plants and animals following the decomposition and transformation of microorganisms and a series of geophysical and chemical processes, it maintains the natural state and biological activity of various structural components and its rich functional groups, such as aromatic rings, conjugated double bonds, hydroxyl groups, carboxyl groups, phenolic hydroxyl groups, carbonyl groups and the like, it allows the soil to be easily agglomerated and granulated. The humic acid may be used for conditioning, treating and renovating soil in view of soil structure, enhancing the degree of soil agglomeration, reducing the ascending speed and height of capillary water, preventing the capillary water from carrying salt content to the ground surface, avoiding a formation of saltmarsh due to moisture drain and salt retention, it is the current research hotspot of rapid and permanent renovation of degraded saline-alkali soils.
However, mineral resource-based humic acid is at now the primary source of humic acid products. Due to the raw material has low content of active ingredients, the solidified active functional groups need to be activated and purified, the process consumes a lot of alkali and acid and is accompanied with a large amount of high-concentration organic wastewater, the production cost is high. How to produce high-activity liquid humic acid at low cost is the key factor and difficult problem for rapid and permanent restoration of saline-alkali land with humic acid.